Flow indicators are frequently installed in water lines, such as automatic sprinkler systems, to provide a signal when the water line is opened, as for example, when a sprinkler head opens or when the water line is ruptured.
In the past, the flow indicators have frequently taken the form of a pivotable paddle or vane which is disposed within the water line. In the conventional type of indicator, the vane is carried by a metal stem that extends through a stuffing gland or seal to the exterior of the indicator. When water flows through the line, the vane and stem are pivoted, and the outer end of the stem acts to trip a switch to actuate an alarm or signal indicating that the water line is open. The stuffing box or seal must be flexible in order to accommodate pivoting movement of the stem and yet under enough tension to prevent leakage at high water pressure. Certain automatic sprinkler systems are designed for a pressure in the range of about 50 psi, while others may be designed for pressures up to 175 psi. Seals are normally designed so that an increase in pressure will increase the sealing effect of the seal, and thus at higher pressures, an increased force is required to pivot the vane and attached shaft that extends through the seal or stuffing box. Thus, in certain situations, where only one or two spinkler heads may be opened, the flow may not be sufficient in a high pressure system to operate the flow indicator.
Over a period of time, the seals age and tend to become hard or brittle and this will change the sensitivity of the indicator, requiring greater water pressure to pivot the vane and generate the signal. Aging of the seal can also promote leakage of the water through the seal.
In an attempt to overcome this problem, it has been proposed in the past to utilize a magnetic switch in which pivoting movement of the vane would actuate the switch to generate the signal or alarm. The U.S. Pat. No. 1,967,017 to Bohner is directed to a flow indicator in which a magnetically susceptible vane, in the static or non-flow position, completes a circuit which is broken when the vane is pivoted by fluid flow within the line.
The U.S. Pat. No. 3,446,986 to Cox describes a magnetically controlled fluid flow indicator utilizing a movable piston which actuates a magnetic switch, while the Hendricks U.S. Pat. No. 1,490,901 discloses a magnetic flow indicator in which the fluid flow moves a sliding core to operate the circuit and generate a signal.